How to diagnose and fix the 25004 inappropriate_isolation_level_for_branch_transaction error code in Postgres.

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The 25004 error code in PostgreSQL signifies an “inappropriate_isolation_level_for_branch_transaction” condition. This error occurs when an attempt is made to set the transaction isolation level in a situation where it’s not permitted, such as within a subtransaction or after a transaction has already performed some operations.

Here’s how to diagnose and fix this error:

  1. Identify the Transaction Block: Review your transaction blocks to find where the isolation level is being set. This error often occurs when trying to set the isolation level after a transaction block has already begun or after a SAVEPOINT has been created.
  2. Set Isolation Level at the Beginning: Ensure that the transaction isolation level is set at the very beginning of a transaction block, before any other commands, including queries, are executed.
  3. Avoid Changing Isolation in Subtransactions: Be aware that you cannot change the isolation level in a subtransaction (after a SAVEPOINT).

Here are some examples of scenarios that might cause this error and how to resolve them:

  • Example 1: Setting Isolation Level After Transaction Start Incorrect usage:
  BEGIN;
  -- Some SQL operations
  SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; -- This will cause error 25004

To fix this, set the isolation level immediately after the BEGIN statement:

  BEGIN;
  SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
  -- SQL operations
  COMMIT;
  • Example 2: Changing Isolation Level After Savepoint Incorrect usage:
  BEGIN;
  SAVEPOINT my_savepoint;
  SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; -- This will cause error 25004

To fix this, set the isolation level at the start of the transaction, before the savepoint:

  BEGIN;
  SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
  SAVEPOINT my_savepoint;
  -- SQL operations
  COMMIT;
  • Example 3: Setting Isolation Level in a Stored Procedure Called Within a Transaction If you have a stored procedure that attempts to set the isolation level, and you call this procedure from within an active transaction, you will get the 25004 error. Incorrect usage within a procedure:
  CREATE PROCEDURE my_procedure() AS $$
  BEGIN
    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; -- This will cause error 25004 if called within a transaction
    -- Procedure logic
  END;
  $$ LANGUAGE plpgsql;

  -- Calling the procedure within a transaction block
  BEGIN;
  CALL my_procedure();
  COMMIT;

To fix this, avoid setting the isolation level within the procedure or ensure that the procedure is not called from within an active transaction.

To avoid the 25004 error, always remember to set the transaction isolation level at the beginning of a transaction block and before any work has been done within the transaction. You cannot change the isolation level once the transaction has started executing commands or within subtransactions.

For more detailed information on transaction isolation and errors, you can refer to resources such as the PostgreSQL documentation on transaction isolation or search for specific error codes like 25004 in the PostgreSQL error codes appendix.

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